News Release Number: STScI-1995-06

Hubble Peers Deep into the Crowded Heart of the Densest Known Star Cluster

The full news release story:

By pinpointing individual suns in the glare of the most tightly packed
cluster of stars in our galaxy, the Hubble Space Telescope has unveiled
hints of either a massive black hole or another remarkable phenomenon:
a "core collapse" driven by the intense gravitational pull of so many
stars in such a small volume of space.

A team of astronomers used the telescope's sharp images to count an
extraordinary number of stars in the ancient globular cluster M15,
about 37,000 light-years away. Hubble spied hundreds of stars in a
tiny area at the center of M15, whereas earthbound telescopes see a
single blur of light. Careful analysis of the distribution of these
and thousands of neighboring stars suggest that at some point in the
distant past, the stars converged on M15's core, like bees swarming to
their hive. This runaway collapse, long theorized by researchers but
never seen in such detail, may have lasted a few million years a
flash in the 12-billion-year life of the cluster.

Thanks to the laws of physics, the core probably stopped collapsing
before many of the stars collided. Rather, stars near the center would
have settled into an uneasy cosmic waltz, both attracted to each other
by gravity and repelled by close encounters that slingshot them through
space.

An alternate scenario also could explain the pileup of stars at M15's
core: a black hole that may have formed early in the cluster's
history. The black hole would have gradually gained mass as more stars
spiraled inward. If it exists, it would now be several thousand times
more massive than our sun.

The study, which will appear in the January 1996 issue of the
Astronomical Journal, was led by Puragra Guhathakurta of UCO/Lick
Observatory, UC Santa Cruz. Coauthors are Brian Yanny of the Fermi
National Accelerator Laboratory, Donald Schneider of Pennsylvania State
University, and John Bahcall of the Institute for Advanced Study in
Princeton. All of the astronomers were associated with the Institute
for Advanced Study when the research began.

A precise reading of the speeds at which stars move near M15's core
would reveal whether the stars are packed so tightly because of the
influence of a single massive object, or simply by their own mutual
attraction. Stars would orbit more quickly in the grip of a black
hole's gravitational field. Such measurements are time consuming but
possible with the Space Telescope.

"It is very likely that M15's stars have concentrated because of their
mutual gravity," Guhathakurta says. "The stars could be under the
influence of one giant central object, although a black hole is not
necessarily the best explanation for what we see. But if any globular
cluster has a black hole at its center, M15 is the most likely
candidate."

The team began using Hubble to observe the centers of globular clusters
in 1991 and now has data on about twenty clusters, but the images of
M15 are by far the most stunning. Hubble's Wide Field Planetary Camera
2 (WFPC2) probed M15 in April 1994, four months after astronauts
installed corrective optics to sharpen the telescope's blurry focus.

"I first started thinking about this observation in 1970," says
Bahcall. "I never expected that Hubble would see things as clearly as
it does. The results are so exciting that they are a dream come true."

Bahcall and astrophysicist Jeremiah Ostriker of Princeton University
first proposed in 1975 that M15 might harbor a black hole. While
distinguished by its extreme density of stars, M15 is in other respects
similar to the rest of the dozens of globular clusters that freckle
space in and around our Milky Way. Each cluster is like a miniature
galaxy, with 100,000 to one million stars in a compact spherical blob.
The largest and closestincluding M15, in the constellation
Pegasusare visible to the naked eye on dark nights as faint hazy
patches.

Globular clusters contain almost no gas or dust and show few signs of
recent star formation. Astronomers believe they are primordial
remnants, left over from the birth of the Milky Way. As such, they are
ideal laboratories for studying how stars evolve. Cluster stars also
provide a limit on the age of the universe, independent of the
expansion of the universe itself.

Stars at the core of M15 may be crowded closer together than anywhere
in the Milky Way except in the galaxy's hidden heart. Attempted
studies of this exotic locale with ground-based telescopes proved
frustrating. Atmospheric blurring washed out the interesting details at
the core. Astronomers used Hubble before its repair mission to examine
M15, but even after correcting the distorted images they could not
discern the true distribution of the innermost stars. In contrast, the
latest WFPC2 photos of the inner 22 light-years of the cluster revealed
about 30,000 distinct stars. That's a fraction of M15's population,
but far more stars than scientists had ever imaged in such a small
region of a globular cluster.

The astronomers used the Planetary Camera (the highest-resolution part
of WFPC2) to study M15's core. The closer they looked toward the core,
the more stars they found. This increase in stellar density continued
all the way to within 0.06 light-years of the centerabout 100 times
the distance between the sun and Pluto.

"Detecting separate stars that close to the core was at the limit of
Hubble's powers," Yanny says. Beyond that point, even Hubble's eagle
eye could not reliably resolve individual stars or locate the exact
position of the core. However, the researchers suspect that stars jam
together ever more tightly inside that radius. The team plotted the
distribution of the stars as a function of distance from the core.
Computer simulations helped them include stars they may have missed
when bright stars drowned out faint ones in the Hubble images. The
resulting pattern matches the predictions of Bahcall and others for
what would happen under the influence of a central black hole. But the
pattern also is consistent with a core collapse, known as a
"gravothermal catastrophe. "Astronomers think the cores of about 20
percent of all globular clusters may have collapsed in this way.

For a gravothermal catastrophe to occur, globular clusters must
transfer energy from the inner parts of the cluster to outer regions.
As this happens, stars near the core lose some of the energy of their
random ("thermal") motions. Several billion years might pass before
the stars become too lethargic to resist the gravitational pull of
their neighbors. At that point, they begin to collapse inward as a
group.

"It's a catastrophe in the sense that once it starts, this process can
run away very quickly," Guhathakurta says. "But other processes could
cause the core to bounce back before it collapses all the way." The
major such process, researchers believe, is the powerful jolt of new
motion that binary-star systems can impart to a third star that wanders
too closeeffectively spreading the stars
out again.

CONTACT

Robert Irion
University of California, Santa Cruz
(Phone: 408-459-2495)